US President Donald Trump has declared a national emergency to bypass congressional approval and secure funding for the construction of his controversial wall on the US-Mexico border.

Trump made the announcement during a press conference in the Rose Garden at the White House on Friday, claiming that the declaration would be necessary to deal with what he described as a crisis of illegal immigration.

“I am going to be signing a national emergency,” Trump told reporters. “It’s a great thing to do because we have an invasion of drugs, invasion of gangs, invasion of people.”

The move, which Democrats have vowed to challenge as a violation of the US Constitution, will potentially divert billions of dollars from other projects such as military construction and drug-interdiction programs to finance the US president’s eight-billion-dollar wall.

During the presser, Trump also predicted that the emergency declaration would will be challenged in federal court but said he would eventually prevail, Presstv reported.

“The order is signed. We will have a national emergency, and then we’ll be sued... and we will possibly get a bad ruling, and we’ll end up in the Supreme Court,” Trump said, anticipating the inevitable Democrat response.

The declaration could infringe on Congress' authority to make major decisions about spending taxpayer funds, a power spelled out as a fundamental check and balance in the US Constitution.

The top two Democrats in Congress, House Speaker Nancy Pelosi and Senate Minority leader Charles Schumer said they would use “every available remedy” to overturn the emergency declaration.

“The president’s unlawful declaration over a crisis that does not exist does great violence to our Constitution and makes America less safe,” they said in a joint statement. “The president is not above the law. The Congress cannot let the president shred the Constitution.”

The US president was also expected to sign later on Friday a spending bill that would avert a second government shutdown set to begin on February 16.

Trump triggered the previous shutdown of about a quarter of the federal government with his December demand for $5.7 billion in wall money. In opinion polls, Thump was widely blamed for the shutdown in the area.